Infrared Spectra of Sodium and Potassium Cyanide

The infrared absorption spectra of sodium cyanide and potassium cyanide vapors have been measured in the region 3500 to 200 cm-1. Two absorptions assigned as fundamental frequencies of the monomeric species were found in each system. The absorption frequencies and their present assignments are: NaCN ν2=238, ν3=2176 cm-1; KCN ν2=207, ν3=2158 cm-1. A third absorption at 2076 cm-1 in sodium cyanide vapor is assigned to the dimer of NaCN. The bond-stretching force constant of the CN group in XCN molecules is sensibly independent of the electronegativity of X. The angle-bending force constant varies directly with the electronegativity of X.